1. Field of the Invention
This invention relates to temperature control in systems wherein a device under test (DUT) is thermally conditioned (heated or cooled to some thermal state) by a thermal conditioning device that adds or removes heat to/from the device by convection, conduction, or radiation. More particularly, this invention discloses a method and apparatus for dynamically determining an optimum temperature profile for the conditioning device such that the DUT is conditioned as quickly as possible without exposing either the conditioning device or the DUT to unacceptable temperatures.
2. Description of Related Technology
The nature of heat transfer is such that a differential in temperature between two masses must exist before heat will flow between them. The greater the temperature differential, the greater the heat flow will be. This phenomenon operates equally for masses that are separate but adjacent and for masses that are adjacent parts of a monolithic whole.
The rate of transfer of heat within a mass having an internal temperature differential is regulated by that substance's resistance to heat flow; its thermal conductivity. Every substance exhibits a different and predictable thermal conductivity.
It follows that to change the temperature of the center of a mass (the “core”) to some desired temperature, the setpoint, the outside surface of the mass (the “skin”) must be exposed to a temperature beyond the desired core temperature for a time period adequate to allow the sufficient transfer of heat given the mass thermal conductivity.
The foregoing concept is clearly illustrated by the everyday example of roasting meat within a conventional oven. The meat is roasted for a given period of time, as determined by its weight, at a comparatively high oven air temperature in order to achieve a desired lower internal or “core” temperature. The differential temperature causes heat to flow to the core of the meat, thereby raising its temperature.
As previously stated, the transfer of heat into or out of the core of a mass consumes a finite amount of time. This time has value, so there is an incentive to achieve the thermal objective (e.g., the desired core temperature) as quickly as possible. A simple solution to accelerating the heat transfer is to increase the temperature differential between the object's skin and it's core. The greater differential will result in faster heat transfer.
However, it will be appreciated that many objects to be heated or cooled have practical thermal limits that must be respected if the object is to be not damaged or destroyed by the heating or cooling process. The most common limits that must be considered are the maximum and minimum temperatures that the skin of the mass can tolerate, and the maximum skin to core temperature differential (thermal stress) that can be tolerated.
Therefore, there is a limit to the amount of heat that can be added or removed from the skin of a DUT during the heating or cooling process without exceeding the thermal limits of the object. Controlling the temperature of the skin of a DUT to that limit will allow the maximum rate of heat transfer to/from the object's core while still respecting the limits of the object's skin. If there is a thermal differential limit as well, then the skin temperature may have to be further restrained to remain within that limit.
Another factor that must be considered is the so-called “latency” of the heating or cooling process. As discussed in greater detail below, if the skin of a DUT is subjected to a more extreme temperature than that desired in the object's core until such time as the core achieves the desired temperature, then the core will be at the desired temperature but the skin will be at a more extreme temperature with the mass between the two areas having a temperature gradient therebetween. If no more heat is added or removed, the entire mass will then equalize in temperature over time. The equalized temperature will be more extreme than the core temperature desired.
Referring again to the example of roasting meat, if a given internal or core temperature is desired, and the meat is roasted at a higher temperature than the desired core temperature, the oven may be turned off when the core temperature has reached a value somewhat less than the desired value. After the oven is turned off, the core temperature will climb to the desired value while the skin region transfers the last of its excess heat to the core in the process of thermal equalization. It should be noted, however, that while this approach may be useful in roasting meat where the allowable tolerances are comparatively high, it is not useful in most thermal conditioning applications having more limited allowable tolerances, and where there is generally little experiential basis for the applying the technique.
To change the core temperature of a DUT undergoing conditioning, the skin of the object is typically exposed to a conductive or convective controlled temperature mass that transfers heat to/from the skin. It is the temperature of this external mass that must be controlled to achieve the desired heat transfer to/from the core of the object. Due to the thermal conductivity and mass of the object there is often substantial thermal latency in the transfer process. One reliable way to achieve the desired core temperature without “overshooting”, is to regulate the skin's thermal environment such that as the object's core approaches the desired temperature the object's skin temperature is forced to approach the same temperature. As the desired temperature is reached, the temperature difference between the core region and skin region approaches zero and heat transfer effectively ceases. See FIG. 1, which illustrates the response of an exemplary prior art thermal conditioning system.
The typical prior art method used for achieving this type of convergent control is to measure the temperature of the thermal environment that acts upon the object's skin and also measure the temperature of the DUTs core. When determining whether to add or remove heat from the thermal environment, it is the average of the two temperatures that is compared to the temperature objective to make the determination. Thus, the environment will be thermally over-driven by the amount the DUTs core varies from the desired temperature. As the DUTs core approaches the desired temperature, the average of the two temperatures will require that the DUTs environment approach the desired core temperature at the same rate.
The temperature averaging method described above has the substantial disadvantage that it has no method for respecting the thermal limitations of the device in which the thermal environment is created, nor does it respect the thermal limits of the DUT being conditioned. It is quite possible for the averaging method to call for additional heating/cooling when either the skin of the device being conditioned, or the conditioning device itself is already at or beyond its limits. Substantial damage to property and risk to operators results from the unrestrained use of such averaging methods.
Therefore, to make effective use of this type of averaging method, it is imperative that the output from the control system that is using the average temperature to call for heating or cooling be restrained if that output calls for the addition or removal of heat in a manner that would cause the limits of the thermal conditioning device, or the DUT, to be exceeded. If the temperature control system is a simple “on/off” thermostat type control, externally restraining the control system output will be satisfactory. However, if the control method being used is a more sophisticated method designed around a closed feedback loop that allows the control system to adapt or modify its control output based upon the results of its prior operation, then the external restraining of the control outputs can be disastrous.
Almost all precision temperature control systems involve a method that uses process result feedback in some type of closed loop to adaptively regulate temperature while adjusting for the thermal response of the environment/device being controlled. Using the feedback data, the controller compares the setpoint to the results observed in the feedback data in order to increase or decrease the controller's output to better achieve the setpoint in the process. The feedback loop, and the analysis of the feedback data over time, is the essence of closed loop temperature process control. Feedback data enables the controller to determine the error in the process control, where error is defined as the difference between the setpoint and actual process temperature at points in time. While there are a multiplicity of control algorithms in use, all generally rely either on periodic error comparison and integration or on periodic comparison of feedback data with expected results data, or on some combination of these. It is the periodic nature of these routines that allows the controller to adjust the level of output.
It is therefore clear that any system that uses such a control method would suffer substantially if its output was externally restrained or “clipped,” since the external clipping of the output would result in substantial variation to the result of the control system's output. To tolerate this kind of modification of the output signal by an external system, the primary control system would have to be fed accurate data as to the magnitude and timing of the clipping. It is an important component of all but the most sophisticated of these closed loop routines that the feedback data be reasonably current. That is, the feedback results most recently obtained are used to adjust the current controller output, thus making the presumption that the control result seen in that data was related to recent output. Additionally, if there is substantial latency in the path from controller output to data feedback, and especially if there are combinations of components with widely varying thermal time constants within the path that provides the feedback data, then the controller receiving the feedback data is deprived of the regular and periodic nature of such data that is requisite for control. Such latency may occur for example when an assembly of items is stacked on a thermal platform, wherein each component of the stack provides significant thermal transmission latency, and wherein each may have a different thermal latency. Upon the top of the stack resides the DUT of interest. Another situation where such latency may occur is that of a large mass DUT which is thermally conditioned by a fluid conditioning system to which the DUT is connected. Hoses or pipes pass the conditioning medium (e.g., fluid or refrigerant) which flows between the DUT and the conditioning unit.
Often, it is possible to obtain temperature feedback data from the underlying platform or thermal conditioning system to control the temperature of the underlying system. However, such control does not compensate for heat gains and losses in the path between the DUT and the controlled device. Controlling the underlying device can provide a stable thermal environment, but seldom will it result in the correct DUT temperature. On the other hand, if the feedback is obtained from the DUT, the thermal latency of the system will result in over driving of the controller outputs that will create an unacceptable controlled temperature oscillation.
Based on the foregoing, an improved method and apparatus for allowing stable control of a significantly latent DUT at the correct stable temperature is needed. Such improved method and apparatus would ideally maintain a stable temperature for the DUT without significant temperature oscillations or hunting.